Fixation tools and devices, which are available in a wide variety of different shapes and sizes, have long been used in the repair of bone defects, such as bone fractures. A physician typically sets the bone to be repaired in the proper position and then uses the fixation tools and devices to secure the bone in that position during the healing process.
A fixation device, such as a bone plate or rod, can be secured to the bone by a fixation tool, such as a bone screw. Alternatively, a bone screw can be used by itself to repair a bone defect. One drawback associated with prior art bone screws is the potential for the bone screw to back out after implantation. To inhibit back out, bone screws have been modified with various thread designs and locking features, with some success.
When installing a bone anchor or screw, a surgeon will typically tap a hole, remove the tap and then install the screw into the hole while maintaining the alignment of the bone with another bone or prosthesis. The bone screw can be secured in the bony bed by filling the hole before installation of the screw with a bone cement, such as polymethylmethacrylate or other fillable and flowable materials.
The use of a solid screw with a bone cement or other fillable material may increase the initial stiffness and strength of the repair, but may not significantly decrease loosening of the screw at the repair site. The substitution of solid screws with cannulated screws that can extrude a bone cement or other fillable material may improve the strength of the repair while at the same time reducing the likelihood that the screw will loosen and pull out, but distribution of the bone cement or fillable material through the screw and throughout the repair site remains a problem. In particular, self-hardening bone cements change viscosity after formulation and the even distribution along the length of the bone screw is challenging for such materials. Another challenge is providing adequate distribution of a bone cement or fillable material while maintaining sufficient screw mechanical strength. Often increasing the number of delivery channels or ports in the screw body can weaken the bone screw. Thus, there remains a need for a cannulated bone screw for use with a bone cement or fillable material that is capable of securing bone at a repair site while also preventing loosening and pull-out of the bone screw following the repair.